The normal human knee is capable of complex motion including flexion/extension, adduction/abduction, anterior femoral-tibial shift, and axial rotation. Successful knee replacement with a prosthesis requires that the bearing surfaces achieve two roles which are contradictory in their physical application.
The first role is that the bearing surfaces and their geometries allow the knee a natural range of movement as determined by the ligaments and soft tissue surrounding the knee. This movement includes flexion/extension, roll back of femur on the tibia and finally rotation of the femur on the tibia at all angles of flexion.
The second role is that the transmission of weight and the friction of movement do not degrade the bearing materials over a reasonable period of time. The aim is that the prosthesis will outlast the patient. To achieve low contact stresses between the bearing surfaces implies a high contact area between those surfaces, ie. a high degree of component congruency. This, in turn, means that there must be a degree of constriction in movement between the bearing surfaces which contradicts the first role mentioned above.
The best solution to date has been the "meniscal knee" developed by Goodfellow and O'Connor in Oxford, England (see U.S. Pat. No. 4,085,466) in 1972. This knee provides a three part articulation comprising a curved femoral surface, a flat tibial surface and a meniscal bearing between the femoral and tibial surfaces. The meniscal bearing is normally formed of a suitable synthetic plastics material such as ultra high molecular weight polyethylene. It has a flat lower surface that slides on the tibial surface and a concave upper surface that receives the femoral surface. Constraint, and the shear stresses that go with it, is generally avoided as the meniscal bearing slides about on the flat tibial surface as the knee flexes, rolls and rotates.
While development of the meniscal knee has been well received there are a number of difficulties still inherent in prostheses of this type. The surgical placement of such prostheses has to be very precise and technical failure for this reason is common. There is also a tendency for the meniscal bearing to dislocate, or spit out, in flexion as the forces applied may be very high.
Other knee prostheses have been developed in which the sliding meniscus is restrained or guided in some way. One variant has curved rails which entrap and guide two sliding menisci, one for each side of the knee joint. A problem with these further variants is that sheer stress transmission, while reduced, is still present and dislocation of the menisci can still occur. Further, when rails are utilised, the result is a highly constrained knee except in the exact geometry of the rails.